This disclosure generally relates to methods for generating aqueous chlorine dioxide solutions. More particularly, the disclosure relates to batch production of chlorine dioxide solutions by adding a solid phase alkali metal chlorite, a solid phase acid, and a solid phase oxidizing agent to an aqueous solution.
With the decline of gaseous chlorine as a microbiocide, various alternatives have been explored, including bleach, bleach with bromide, bromo-chlorodimethyl hydantoin, ozone, and chlorine dioxide (ClO2). Of these, chlorine dioxide has generated a great deal of interest for control of microbiological growth in a number of different industries, including the dairy industry, the beverage industry, the pulp and paper industries, the fruit and vegetable processing industries, various canning plants, the poultry industry, the beef processing industry and miscellaneous other food processing applications. Chlorine dioxide is also seeing increased use in municipal potable water treatment facilities and in industrial waste treatment facilities, because of its selectivity towards specific environmentally-objectionable waste materials, including phenols, sulfides, cyanides, thiosulfates, and mercaptans. In addition, chlorine dioxide is being used in the oil and gas industry for downhole applications as a well stimulation enhancement additive.
Unlike chlorine, chlorine dioxide remains a gas when dissolved in aqueous solutions and does not ionize to form weak acids. This property is at least partly responsible for the biocidal effectiveness of chlorine dioxide over a wide pH range. Moreover, chlorine dioxide is a highly effective microbiocide at concentrations as low as 0.1 parts per million (ppm) over a wide pH range.
The biocidal activity of chlorine dioxide is believed to be due to its ability to penetrate bacterial cell walls and react with essential amino acids within the cell cytoplasm to disrupt cell metabolism. This mechanism is more efficient than other oxidizers that “burn” on contact and is highly effective against legionella, algae and amoebal cysts, giardia cysts, coliforms, salmonella, shigella, and cryptosporidium.
Unfortunately, chlorine dioxide in solution is unstable with an extremely short shelf life and thus, is not commercially available. Chlorine dioxide solutions must typically be generated at its point of use such as, for example, by a reaction between a metal chlorate or metal chlorite in aqueous solution and a liquid phase strong acid. However, the use of liquid phase strong acids poses handling issues and safety concerns. In view of this, it would be desirable to develop point of use systems that do not employ liquid acids.
Electrochemical processes provide a means for continuously generating chlorine dioxide for point of use applications. One difficulty with electrochemical processes is that it can be difficult to control the generation of undesirable species. Moreover, the electrochemical processes generally require a power source and an electrochemical apparatus, which can be relatively expensive, can require a large footprint, and can require plumbing configurations with the source to be treated with the chlorine dioxide.
Chlorine dioxide has also been continuously produced from a chlorine dioxide precursor solution by contacting the precursor solution with a catalyst (e.g., catalysts containing a metal such as described for example in U.S. Pat. No. 5,008,096) in the absence of an electrical field or electrochemical cell. However, it has been found that the support materials for the catalytic sites tend to quickly degrade due to the oxidizing nature of chlorine dioxide. Moreover, the continuous processes are not effective for batch production.
Accordingly, there is a need for an economical batch process and composition for generating chlorine dioxide that does not pose safety concerns.